The present invention relates, in general, to systems and apparatus for generating electrical power within a borehole, and, more particularly, to a downhole alternator for generating power upon rotation of the portion of the drillstring in which it is mounted.
When drilling boreholes in the earth's crust for such purposes as oil and gas exploration, tunneling under obstacles for the placement of cables, or for a wide variety of other purposes, it is generally necessary to provide drilling tools in the drillstring which incorporate various sensors and control instrumentation for guiding the direction of drilling, detecting the conditions at the drill head, and transmitting data and control signals between the drill head and the earth's surface. Such downhole instrumentation requires a reliable source of electrical power for operation, and over the years a great deal of effort has been put into developing such a source. For example, attempts have been made to supply electrical power to downhole instruments from a surface source, as by way of wires or cables extending the length of the drillstring. However, the use of such wires or cables in rotating drillstrings has been unsatisfactory, because of the difficulty in maintaining secure connections at each joint in the drillstring.
Downhole battery supplies have generally replaced surface power sources for downhole circuitry, but they, too, have limitations in that they have a limited life, requiring periodic replacement that results in costly down time for the drilling operation.
Various attempts have been made to overcome these problems by introducing a downhole electrical generator to either recharge batteries or to provide power directly to the downhole instruments. Such generators have been powered by the flow of drilling fluid in the borehole, but difficulties have been encountered in maintaining reliable operation in the hostile environment of downhole drilling. The abrasive, high-pressure drilling fluid flowing through and around the drillstring is destructive of generator moving parts, and is particularly hard on rotating seals, while the placement of such generators in the flow path of the drilling fluid interferes with the flow and prevents access to the drillstring below the generator location.